A dip tank presents the most straightforward and the most unforgiving version of a Class B hazard: a large, continuously exposed surface of flammable or combustible liquid, open to the room, with parts moving in and out of it. There is no atomization and no enclosure to complicate the picture. The fuel is simply there, in quantity, with a free surface available to ignite across its entire area.
That geometry is why NFPA 34 sizes protection from the liquid surface area rather than from the room. A fire on a dip tank involves the whole surface almost immediately, and the agent has to establish coverage across all of it at once. Undersizing shows up not as reduced performance but as a fire that reestablishes itself on the uncovered portion of the tank seconds after discharge ends.
The secondary hazards are drainage and vapor. Coating that drips off parts onto the drainboard and floor extends the fuel footprint outside the tank outline, and heavier-than-air solvent vapor collects in tank pits, floor trenches, and below-grade drainage. A protection scheme that addresses the tank surface and ignores where the liquid and its vapor actually go is incomplete in a way that becomes obvious only during an incident.